1. Field of the Invention
The invention relates to a process for producing a substrate which has a metal, glass or ceramic surface and has been provided with a glasslike layer comprising interference pigments, the substrate obtained and its use.
2. Discussion of Background Information
Nanoglass layers are described in WO 2005/066388 and WO 98/45502. They are consolidated at relatively high temperatures, which are, however, far below the transformation temperature of the glass in question, on metal substrates such as steels, aluminium or brass. The lower consolidation temperature possible compared to the information in the literature arises from the nanoscale structure of the layer applied from the liquid phase, which causes a high internal thermodynamic energy and which consequently lowers the consolidation temperature. This allows transparent, glasslike layers of very low thickness (e.g. 3 to 10 μm) to be obtained on the metal, glass and ceramic surfaces mentioned. These layers are dense and mechanically very stable, but, in contrast to molten glasses, have a certain degree of plastic deformability.
It is also possible in principle to provide the liquid coating materials with thermally stable pigments. In that case, no longer transparent, but rather coloured, glass layers arise, which serve as a matrix for the pigments.
To produce well-defined layers, the pigment diameter generally must not exceed one tenth of the layer thickness, since smooth layers otherwise cannot be obtained and the pigments project from the surface. This means that, to produce impeccable nanoglass layers, the pigment diameter, according to the layer thickness, must not exceed 2 to 5 μm. Common pigments are, however, generally obtainable only with diameters in the micrometer range. This is for various reasons:                The comminution of the pigments to particle sizes below 1 μm is additionally very complicated and is not possible in many cases.        Very fine pigments generally cause a different colour effect (brightening) from corresponding pigments with diameters above 1 μm.        
This finding is in principle valid for all types of pigments, i.e. even for pigments which are not based exclusively on the electronic absorption of parts of the electromagnetic spectrum, but rather on the interference of dielectric layers matched to one another with different refractive indices, optionally in combination with absorption effects. In the case of such interference pigments, compared to conventional pigments, there is, however, the further problem that, in the case of mechanical comminution, the interference layers of the pigments are damaged or even destroyed, such that the colour effect suffers or is even generally destroyed completely. The interference pigments currently available on the market have a diameter of generally significantly more than 10 μm. For the reasons mentioned above, manufacturers of interference pigments state that it is not possible to subject these pigments to a comminution process.
It is possible in principle to produce the above-described nanoglass layers with such interference pigments, but, owing to the relative size of these interference pigments, the resulting layers are rough for the reasons explained at the outset, some of them are cracked, and they are generally not impervious and/or contain pores as a result of the consolidation problems caused by the pigments. On the other hand, such nanoglass layers comprising interference pigments are very attractive for decorative reasons, since it would thus be possible for the first time to provide layers based on nanoglass with these luster pigments and open them up to an application.
The inventive objective consisted in producing nanoglass layers with incorporated pigments based on interference, without the abovementioned disadvantages in relation to the layer properties occurring, and the customary interference pigments which are commercially available should be used as a starting material. At the same time, the extent to which the interference pigments are also suitable for the achievement of layers with low roughness on other inorganic surfaces too, for example glass surfaces and ceramic surfaces, should be examined.
The object of the invention is surprisingly achieved by virtue of it being possible, with the aid of a wet grinding process, particularly by virtue of the use of a wet ball grinding process, and especially with use of a high-speed rotary ball mill with a rotor and stator, to grind the pigments sufficiently gently that, firstly, the interference effect was maintained virtually completely, and, secondly, the particle size was reduced such that qualitatively high-value nanoglass layers comprising interference pigments were obtainable.